In copending commonly-assigned U.S. patent application Ser. No. 447,666, now U.S. Pat. No. 3,949,936 filed on Mar. 4, 1974, apparatus and systems are disclosed for imposing time limits on the firing of steam boilers for residences and the like wherein fuel consumption is limited to specified periods by connection of a power supply to a boiler firing device through a series circuit having a time-controlled switch arrangement selectively closed by electromechanical clock mechanisms and a switch having state responsive to boiler steam heat content. In their specified usage, such previously known apparatus and system are directed to steam environments and employ, as their switch responsive to boiler steam heat content, such as the customary steam pressure-sensitive switch closeable at a first stream pressure level and thereafter not openable until a second higher steam pressure level is reached. In their direction, such known apparatus and system provide that boiler firing shall occur only where both the time-controlled and steam pressure responsive switches are closed, i.e., during only time-selected portions of periods when the boiler firing devices would otherwise be energized.
Other diverse types of heating control systems employing time intervention in the customary direct control of a heat generator by an area heat sensor are set forth in U.S. Pat. No. 2,175,945 to Simpson and U.S. Pat. No. 2,162,116 to Peltz. In the heat control system of the Simpson patent, a power supply is connected to a heat generator through a series circuit comprising a room thermostat and a time-controlled switch. The time-controlled switch is in the form of an electromechanical commutator, segments of which periodically engage a fixed brush member to complete the energizing circuit for the heat generator. The commutator segments are of fixed angular extent and of fixed disposition relative to one another whereby the periodicity of connect-disconnect cycles is uniform. Further, the commutator shaft is driven at a constant speed preselected in accordance with outdoor temperature and values calibrated from steam radiator and like system components. The system involved in the Peltz patent operates to introduce a fixed delay in reporting thermostat demands to the heat generator. Thus, on the generation of a heat demand by the thermostat, a time delay mechanism is energized and, after the time-out thereof expires, a normally-open switch is closed conveying the heating demand to such as a stoker motor. As in the case of the Simpson patent, the time intervening mechanism is electromechanical and of fixed time-defining nature.
Of the three varieties of prior art systems above-discussed, only that of the referenced copending application provides for ready variation of the periods of time intervention in heating control systems. The latter two systems are evidently limited by reason of their lack of such time varying facility and the system of the referenced copending application has disadvantage by reason of its electromechanical structure and its specified applicability only to steam related systems and steam heat content measuring requirement.